Conventionally, with respect to two-dimensional image detectors for detecting images by using radioactive rays, there have been known a radioactive-ray two-dimensional image detector in which semiconductor sensors, each of which generates charges (electron-positive hole) upon sensing X-rays, are arranged in a two-dimensional format and electric switches are respectively attached to these sensors so that the electric switches are successively turned on for each row so as to read out charges of the sensors for each column. Specific structures and principles of such a two-dimensional image detector are described, for example, in documents "D. L. Lee, et al., `A New Digital Detector for Projection Radiography`, SPIE, 2432, pp. 237-249, 1995", and "L. S. Jeromin, et al., `Application of a-Si Active-Matrix Technology in a X-Ray Detector Panel`, SID 97 DIGEST, pp. 91-94, 1997", and Japanese Laid-Open Patent Application No. 342098/1994 (Tokukaihei 6-342098) (Published on Dec. 13, 1994).
The following description will discuss the structure and principle of the above-mentioned conventional radioactive-ray two-dimensional image detector. FIG. 7 is an explanatory drawing that schematically shows the structure of the above-mentioned radioactive-ray two-dimensional image detector. Moreover, FIG. 8 is an explanatory drawing that schematically shows the construction of each pixel of the above-mentioned radioactive-ray two dimensional image detector.
As illustrated in FIG. 7 and FIG. 8, the radioactive-ray two-dimensional image detector is constituted by an active-matrix substrate that is a glass substrate 51 on which XY-matrix electrode wiring (gate electrodes 52 and source electrodes 53), thin-film transistors (TFT) 54, and charge-storage capacitors (Cs) 55, etc. are formed, and a photoconductive film 56, a dielectric layer 57 and upper electrode 58 that are formed virtually on the entire surface of the active-matrix substrate.
The charge-storage capacitor 55 has a construction in which charge-storage capacity electrodes (Cs electrodes) 59 and pixel electrodes 60 connected to the drain electrodes of the TFTs 54 are aligned face to face, with an insulating film 61 located in between.
The photoconductive film 56 is made from a semiconductive member that generates charges (electron-positive hole) upon irradiation by radioactive rays such as X-rays. In accordance with the above-mentioned documents, amorphous selenium (a-Se), which has a high dark resistivity so that it exhibits a superior photoconductive property to X-ray irradiation, is adopted. The photoconductive film 56 is formed by the vacuum deposition method with a thickness of 300 to 600 .mu.m.
Moreover, active-matrix substrates, which are formed during processes in manufacturing liquid crystal displays, can also be adopted as the above-mentioned active-matrix substrate. For example, an active-matrix substrate, used for an active-matrix-type liquid crystal display (AMLCD), has a construction having TFTs, XY-matrix electrodes and electric storage capacitors formed by amorphous silicon (a-Si) and polysilicon (p-Si). Therefore, those active-matrix substrates, which are formed during processes in manufacturing liquid crystal displays, are readily utilized as the active-matrix substrate used for a radioactive-ray two-dimensional image detector with slight designing modifications.
The following description will discuss the operation principle of a radioactive-ray two-dimensional image detector having the above-mentioned structure.
When the photoconductive film 56 such as an a-Se film is irradiated with radioactive rays, charges (electron-positive hole) are generated in the photoconductive film 56. As illustrated in FIGS. 7 and 8, since the photoconductive film 56 and the charge-storage capacitors 55 are electrically connected in series with each other, when a voltage is applied between the upper electrode 58 and the Cs electrode 59, charges (electron-positive hole) generated in the photoconductive film 56 respectively shift toward the positive (+) electrode side and the negative (-) electrode side, with the result that a charge is stored in each charge-storage capacitor 55.
Here, a charge-blocking layer 62 consisting of a thin insulating layer is formed between the photoconductive film 56 and the charge-storage capacitors 55, and this serves as a blocking type photodiode that blocks injection of a charge from one side.
With the above-mentioned function, the charges stored in the charge-storage capacitors 55 can be drawn outside from source electrodes S1, S2, S3, . . . , Sn, by making the TFTs 54 in an open state by using input signals of gate electrodes G1, G2, G3, . . . , Gn. Since the electrode wiring (the gate electrodes 52 and source electrodes 53), the TFTs 54, the charge-storage capacitors 55, etc. are all installed in a XY-matrix format, image information of X-rays can be two-dimensionally obtained by scanning signals inputting to the gate electrodes G1, G2, G3, . . . , Gn in a line sequential manner.
Additionally, in the case when the photoconductive film 56, used in the two-dimensional image detector, exhibits photoconductivity not only to radioactive rays such as X-rays, but also to visible light rays and infrared rays, the two-dimensional image detector also functions as a two-dimensional image detector for detecting images resulting from visible light rays and infrared rays.
The radioactive-ray two-dimensional image detector for detecting images by using radioactive rays is designed to use a-Se as the photoconductive film 56. However, since a-Se has a dispersion-type conductive property to photocurrents which is inherent to the amorphous material, it has a poor response characteristic. Moreover, since a-Se does not have a sufficient sensitivity (S/N ratio) to X-rays, information can not be read until just after the charge-storage capacities 55 have been charged sufficiently through irradiation by X-rays for a long time.
Moreover, in an attempt to prevent a charge from being stored in the charge-storage capacitors 55 due to leakage current and to reduce a leak current (dark current) upon irradiation by X-rays, there is an dielectric layer 57 installed between the photoconductive film (a-Se) 56 and the upper electrode 58. Since a charge remains in this dielectric layer 57, a sequence needs to be added so as to eliminate the residual charge for each frame. The resulting problem is that the two-dimensional image detector is only used for picking up still images.
In order to obtain image data from motion images, it is necessary to use a photoconductive film 56 made from a photoconductive member which is a crystal material and also has a superior sensitivity to X-rays. The improvement of the sensitivity of the photoconductive film 56 allows the charge-storage capacitors 55 to be sufficiently charged even irradiation of X-rays for a short period, thereby eliminating the necessity for applying a high voltage to the photoconductive layer 56 and consequently eliminating the dielectric layer 57. For this reason, it is not necessary to add the sequence for eliminating the residual charge for each frame, and it becomes possible to meet the demands for motion images.
With respect to photoconductive members which have a superior sensitivity to X-rays, CdTe, CdZnTe, etc. have been known. In general, the photoelectric absorption of X-rays is directly proportional to the effective atomic number of an absorbing material raised to 5th power; therefore, for example, supposing that the atomic number of Se is 34 and that the effective atomic number of CdTe is 50, the sensitivity is expected to be improved to approximately 6.9 times. However, when an attempt is made to use CdTe or CdZnTe as the photoconductive film 56 of the radioactive-ray two-dimensional detector instead of a-Se, the following problems are raised.
In the conventional application of a-Se, the vacuum vapor deposition method can be used as the film-forming method, and since the film forming temperature is normal temperature, film formation is easily made on the above-mentioned active-matrix substrate. In contrast, in the case of CdTe and CdZnTe, film-forming methods such as the MBE method and the MOCVD method have been known, and in particular, the MOCVD method is suitable from the viewpoint of film formation to a substrate with a large area.
However, in the case of the film formation of CdTe and CdZnTe by using the MOCVD method, with respect to its materials, organic cadmium (DMCd) has a thermal decomposition temperature of approximately 300.degree. C., and organic telluriums (DETe and DiPTe) have respective thermal decomposition temperatures of approximately 400.degree. C. and 350.degree. C.; therefore, a high temperature approximately 400.degree. C. is required for the film formation.
In general, the above-mentioned TFTs 54, formed on the active-matrix substrate, use an a-Si film and a p-Si film as semiconductor layers. In order to improve the semiconductor characteristics, the a-Si film and the p-Si film are formed while adding hydrogen (H.sub.2) at a temperature approximately 300 to 350.degree. C. The TFTs 54 formed in this manner have a heat resistance of approximately 300.degree. C., and when the TFTs 54 are processed at a temperature exceeding this temperature, hydrogen tends to be released from the a-Si film and the p-Si film, thereby causing degradation in the semiconductor characteristics.
Therefore, it has been difficult in practice to form a CdTe film or a CdZnTe film on the active-matrix substrate by using the MOCVD method, from the viewpoint of film-forming temperature.